The present invention relates to a novel method and apparatus for the non-invasive monitoring of arterial blood pressure waves.
The arterial blood pressure is widely used for assessing the status of a subject's cardiovascular system, and its measurement is one of the most common clinical procedures practiced in all levels of medicine for both diagnostic and prognostic purposes. The direct measurement of blood pressure involves the introduction of a catheter in an artery of the body. Because of its invasive nature, this technique is usually restricted to situations where it is essential, for example, in certain surgical procedures and in the intensive care of critically ill patients. Accordingly, most blood pressure measurements are made indirectly by the use of a cuff.
The sphygmomanometric auscultatory method, which was first described and used about 80 years ago, is by far the one most commonly used today. This method is based on the application of an external pressure around an artery (in the arm), and the monitoring of the mechanical pulsations, thus modified, at a point below the constricted area. The pressure is applied by inflating a rubber bladder surrounded by a cuff placed on the arm. The pressure is monitored by a mercury or annular manometer, while the pulsations are monitored by a stethoscope. Automatic measurement devices use the same principle with the exception that the pulsations are monitored by other means, such as a microphone, piezoelectric sensor, photoelectric sensor, more sophisticated movement detectors based on the Doppler principle, etc. By recognizing specific changes in the nature of the mechanical vibrations or sounds, the operator or automated device can determine the systolic, diastolic, and the mean blood pressure in the artery.
The above known indirect measurement techniques suffer from a number of inherent drawbacks, including the following:
1. The error in the determination of the systolic and diastolic pressures, which error commonly exceeds +-10 mm Hg, is a function of the ability and experience of the operator, the relationship between the arm diameter and cuff dimensions, the pressure value, the degree of peripheral vascular sclerosis, etc. PA1 2. The change in sound, by means of which the end point or actual pressures are being determined, are difficult to detect. PA1 3. Shortly after the cuff is inflated, pain ensues in the extremity. PA1 4. Cuffs of different sizes should be used for different patients. PA1 5. The procedure of applying the cuff is inconvenient. PA1 6. The method cannot be used for continuous blood pressure measurements and is inconvenient even for infrequent pressure determinations. PA1 7. The method can hardly be used for pressure determination while the patient is pursuing his normal activities. PA1 8. The method requires trained personnel to carry out the measurement. PA1 9. The method is inconvenient for self use. PA1 10. The method is not easy to automate. PA1 11. The measurement procedure requires a relatively long time, e.g., 30-60 seconds.